Gamepad alone probably doubled the price of the thing.
If it had launched with a normal controller, they could have halved the price of the unit.
Resulting in much more sales overall (imo).

The gamepad was bulky, heavy, and not optimal to use (most people resorting to buying a pro controller for it, to game with).
And in the games were they really tried to take advantage of the gamepad, the games suffered for it.
(starfox comes to mind)

Anyways, bad sales caused Nintendo to pull developement early on, so there was game droughts, that just made the issue worse.
So it wasnt all just hardware's fault. It was a spiral of alot of things going wrong.

People that say it was just "marketing" are wrong however.
Nintendo brand is so big (so many loyal fans), that even with horrible marketing, if they put out a good device it will sell reguardless.
For people to not buy the Wii U, it was apparent that it wasnt a great product.

It came out, near the end of the gen, and was basically just upto par with the others on the market (that had been out for along time, and was soon moveing on). Soon PS4/XB1 launched, and that took alot of the hype away. At that point it was clear, the Wii U was old gen tech.

Hence another reason I think Nintendo purposefully launches their consoles, mid-gen, away from its competitors.
And why it changed its hardware to be differnt, so it would avoid direct compairsions.

Marketing and name killed it. The lack of software was a result of poor sales.

I liked how easy it was to backup files on WiiU. All that was taken away with the Switch lol

I also liked how you could play COD with a mate at home and just chat. One on screen and one on TV

The hardware. Its was sooo weak of a console. Wasn't a big jump from the wii or even xbox 360. Apart from that everything was great.

The Wii U's library is actually pretty good, if somewhat on the small side.

Pikmin 3, Super Mario 3d World, the Zelda HD remakes, Super Mario Maker, Mario Kart 8, Splatoon, Bayonetta 2, Xenoblade X, DKCTF, Smash 4, Hyrule Warriors, Yoshi's Wooly World, Rayman Legends, and much more. Many of these are genre defining and among the best games of their respective years. In 2014 the general consensus was that the Wii U actually had the best lineup of the HD systems since the PS4 and XB1 had such a disappointing year software-wise.

It took a while to get there though. The game drought of early 2013 really hurt the system right after launch and killed any momentum it might have had. Had Nintendo had games ready for release in those first 6 months of 2013 instead of relying on just Lego City and a remake of the Wii's Monster Hunter they could have done much better, but HD development was still very new for them.

The terrible marketing was an even bigger problem for Nintendo than the lack of games in 2013. They focused so much on the gamepad that the general public was confused as to whether it was a console or just a peripheral, and the gamepad lacked the immediate appeal and ease of understanding the Wiimote has.

The hardware was a problem in the sense that it was difficult for third parties to develop for and overpriced for what it was. The GPU was the best part and much better than what the 360 and PS3 had, and the system had about double the usable RAM. But the RAM was slow and the CPU was pathetic, both really bottlenecking the system. If the system had sold better third parties would have had more reason to put the effort into working around those bottlenecks, but with its poor sales there was no incentive to put in the extra work for games that wouldn't sell. The Wii U's specs also did not justify a $300-$350 price tag even though Nintendo was actually selling it at a loss. It seems that the gamepad and all its bells and whistles were what pushed the price so high, and that price never went down. A Wii 2 with the same specs and a focus on the Wiimote again would have been much cheaper and sold much better, Also, a system with better specs but no gamepad would have been easier to develop for, more worth the price, and also sold much better. In both cases the lack of the gamepad would have automatically improved the marketing by forcing Nintendo to focus on the console.

The gamepad itself is what doomed the system. Without it there would have been no confusion, the system could have been either cheaper or more powerful, the marketing would have been better by default, and it would have been able to capitalize on more of the Wii's market. The early 2013 drought may still have happened, but there likely would have been more third party games in that period under such a scenario like Rayman Legends, which was delayed to be released on other platforms when the Wii U started to fail.

It may not have been a big jump from the 360 but it was a big jump from the Wii. 

Wii was a 480p system with 88MB of RAM, a Direct X7 era GPU, and a single core 729MHz PowerPC750 CPU.

Wii U was a 720p system with 1GB of RAM, a Direct X10 equivalent GPU, and a triple core 1.24GHz PowerPC750 CPU.

Gamepad should've been peripheral (with much better range), so that CPU/GPU/RAM can be much beefier.

Hardware wise it was relatively expensive whilst being  very dated and the gamepad was simply not a compelling gimmick. Just as the 360/PS3 era was ending Nintendo was playing catch up with a Wii branded HD console but without any of things that made those systems a success like next gen technology, or the promise of a new generation of third party games.

Nor did they have what makes a Nintendo system great; compelling first party titles. Sure its end of life library was pretty solid but getting there took a while and it had the biggest game droughts we've seen in modern console history. Meanwhile the Wii brand was itself dead in the water and considered fad-like.

The Switch took 4 years of Wii U software efforts and repackaged them into portable, sleeker, more technologically exciting package. Targeted Nintendo's core core audience and delivered a really great piece of tech with software support released in quick succession to show it off. If the Wii U launched with Breadth of the Wild and Mario Kart 8 I'm sure it would have sold at least double what it actually did. I can't stress enough that the Switch was also boasting pretty amazing hardware, they could of gone for a more powerful chip but what is in it was still impressive for something so small in 2017. Even on more powerful mobile devices you were simply not going to get something that BOTW running on it because they lacked the dedicated software.

The tablet controller was okay (within its small wireless range), but I disliked its abysmal battery life... and the whole system depended on that battery life: When the tablet controller shuts down, the main console also shuts down.

Fixed it with the "high capacity battery" periphal... but Nintendo should have put that battery in all WiiU tablet controllers.

For me, it was simply underpowered. Apart from that, nothing major really. I wish the screen was larger, considering how big the gamepad was and I'm still salty about the Nintendo's decision to ditch analog triggers (why Ninty, why!), but that was more of the Wii thing than anything else.

The game library was pretty bad too which didn't help (Wii U games are selling well on the Switch, but the Swtich has those games AND the Switch exclusive ones, and probably wouldn't be selling well with just the Wii U library), but the Gamepad simply didn't offer anything worthwhile. Most games used it only for off TV play, and maybe displayed a map on it. Turns out people weren't all that interested in a PS360 level console that you can play without a TV, but only if you're within 20 feet of the base.